Partial Lunar Eclipse
When the Earth moves between Sun and Moon, its shadow falls onto the Moon, resulting in a total lunar eclipse. When Sun, Earth and Moon are not precisely aligned, only a fraction of the Moon's visible surface moves into the Earth's shadow. This phenomenon is called a partial lunar eclipse.
Partial lunar eclipses explained
Although the Moon is a dark object, it can be seen in the sky most of the time because its surface reflects the Sun's rays back to Earth. A partial lunar eclipse occurs when the Earth moves between the Sun and Moon but the three celestial bodies do not form a perfectly straight line. When that happens, a fraction of the Moon moves into the darkest, central part of the Earth's shadow (umbra) and does not receive any direct sunlight. The other part of its visible surface is within the shadow's much brighter outer part (penumbra).
Unlike solar eclipses, which can only be seen along a narrow path on Earth, partial eclipses of the Moon can be observed all across the night side of Earth because observers are situated on the same celestial body that casts the shadow. For this reason, the probability to witness a lunar eclipse from any one point on Earth is much higher compared to solar eclipses, even though both occur in similar intervals.
What does it look like?
During the eclipse, the Earth's shadow slowly grows across the Moon's surface until it reaches its greatest magnitude. After this high point, the shadow diminishes again. The eclipsed part of the Moon is still visible as a dark yellow, orange or brown entity. Although the Earth blocks all direct sunlight from that part of the Moon's surface, some rays still find their way via the Earth's atmosphere.
When does a partial lunar eclipse occur?
A partial lunar eclipse can be observed at night and during Full Moon when
- the Moon is near one of its orbital nodes so Sun, Earth and Moon roughly form a straight line,
- and the observer is located on the night side of Earth.
The Moon's orbit and lunar nodes
The Earth revolves around the Sun and the Moon circles the Earth. During Full Moon, the Earth passes roughly between Moon and Sun. However, in most cases the three celestial bodies do not form a completely straight line, so the Moon is not eclipsed.
The reason why lunar eclipses do not happen every Full Moon is that the lunar orbital plane - the imaginary flat surface whose outer rim is formed by the Moon's path around Earth - runs at an angle of approximately 5 degrees to the Earth's orbital plane around the Sun (ecliptic). The points where the two orbital planes meet are called lunar nodes. Only if the Moon appears near one of the two lunar nodes during Full Moon can a lunar eclipse be observed from the Earth's night side.
The type and magnitude of the lunar eclipse depends on how precisely Sun, Earth and Moon line up. A partial lunar eclipse can be observed if the three form an almost straight line.
The Earth's shadow
Like any other object's shadow, the Earth's shadow consists of three different areas: the innermost and darkest part (umbra), the lighter, outer part (penumbra), and a partly shaded area beyond the umbra (antumbra). During a partial lunar eclipse, parts of the Moon pass through the Earth's umbra, while the remaining portion of its visible surface is within the penumbra.
Did you know...?
The size of the eclipsed portion of the Moon's surface (magnitude) is the same irrespective of the observer's location on the night side of Earth. However, because observers on the southern hemisphere stand “upside-down” compared to observers on the northern hemisphere, they also see the Moon “upside-down”. The orientation of a lunar eclipse and the direction in which the shadow appears to move across the Moon's surface can therefore vary according to latitude.